How to Measure Water Levels in Wells, Rivers, and Lakes
Measuring water levels in a body of water is not hard to do. A tape measure or measuring stick will work just fine. But these water level measurements are often needed on a regular basis to track changes in water levels from one season to another, or even every minute. Luckily there are a wide range of systems for accurately and precisely measuring water levels over time in different conditions. This blog should help you decide which system is best for your needs.
Water Level Pressure Transducers
Water has a known weight and, therefore, produces a known pressure based on the depth of the water. Pressure transducers accurately measure this pressure, which is easily converted to the height of water above the transducer. So long as the transducer is always located at the same depth and the elevation of the pressure transducer is known, it is easy to obtain an elevation measurement of the water level. Pressure transducers are connected to a datalogger that records and stores the information transmitted by the pressure transducer.
Advantages
Pressure transducers are only about the size of a dry erase marker and can be attached to very long cables, making them an excellent choice in deep wells. Pressure transducers can also effectively be used in surface water situations, such as measuring the depth of water in a stream or at a primary or secondary flow measuring device (see our previous LWS blog on the differences between primary and secondary measurement devices). They also don’t require external power sources and can operate for years on a single set of AA batteries. Pressure transducers have the largest collection frequency range and can be easily programed to collect readings many times per second, up to a year or more apart. They are also one of the lower cost options on this list due to the lack of system complexity or moving parts.
Disadvantages
While not an issue when installed in groundwater monitoring wells, when used in a lake or a stream, pressure transducers can easily be damaged by ice, so care must be taken to either remove the transducer under freezing conditions or set it to an adequate depth so it is not subject to freezing. A pressure transducer measures data using a strain gage and freezing will break the strain gage.
Another issue with pressure transducers is that changes in barometric pressure must be accounted for to obtain accurate readings. Day to day changes in barometric pressure can cause errors in water level readings by a foot or more. Barometric pressure correction is most often done with either a vented cable on the transducer or using a second pressure transducer measuring barometric pressure above the water surface. Corrosive water can also cause problems over time since the sensor is fully submerged.
Water Level Bubblers
Bubble system schematic (Source: USGS).
Water level bubblers work by measuring the pressure difference between the atmosphere and a fixed point below the water surface. A long hose from the bubbler is fixed at a point on the bed of a stream, or down a well, and pressurized air is pumped into the hose. The pressure in the hose rises until the air can escape, or “bubble”, out of the hose and the pressure stops rising. This air pressure needed to make bubbles exit the hose is easily converted to the depth of the water above the fixed end of the hose.
Advantages
The forcing of air through the pressure hose makes bubblers self-clearing of materials like silt or sand on a riverbed (as long as the bubbler is not set such that excessive sedimentation affects the bubbler), making them ideal for measuring water levels in large rivers. While river ice will impact the accuracy of the water level readings, ice will not damage the bubbler system itself.
Disadvantages
Bubblers are complex systems requiring external power, an air compressor, a pressure transducer, and must have airtight fittings. However, with a simple solar panel, bubblers can be reliably deployed in very remote and inhospitable locations. This complexity also makes them the most expensive option on this list.
1 psi is equal to 2.31 feet of water, so for deep wells with hundreds or thousands of feet of water a standard air compressor can’t produce enough pressure. High pressure nitrogen gas bottles are more commonly used as a gas source in deep wells.
Ultrasonic Sonar Water Level Measurement
Sonar gage schematic.
Sonar-based water level measurement systems are conceptually very simple. The sonar device is mounted to a static location above the water, such as a bridge. The sonar device measures the water level by emitting a sound wave and measuring the time for the signal to bounce off the water and return to the device, giving a water level based on the distance from the fixed mounting location. They can be used in surface water applications and wells over 1,000 feet deep. The cost of sonar-based water level measurement systems is largely related to the maximum distance from the sensor to the water. The greater the distance, the greater the cost. These systems can range from on the low end for this list, or on the high end.
Advantages
Sonar systems are small, accurate, and unaffected by the conditions on the bottom of water channels. Since they do not touch the liquid, they obtain accurate readings regardless of the density of the liquid, for example, fresh water versus sea water.
Disadvantages
Sonar systems require calm water to obtain consistent measurements, so mounting them over rapids would not be advised. Sonar systems only measure the distance to a surface, not just water. This means they can be thrown off by ice, floating trash, or boats, and the sonar signal can be attenuated by snow, rail, or dust. The mounting point must truly be static. Some bridges often flex with heavy traffic loads or even wind, so be picky about your mounting location. Sonar systems have limited ranges compared to bubblers, or pressure transducers, making them best suited to well controlled environments.
Float-Based Water Level Measurement
Flume float in dry stilling well.
While limited in their applications, a simple float is a very reliable way to measure water levels. Float systems are mounted to a fixed location above the water surface and have a large pulley on an encoded shaft. A cable runs over the pull with a weighted float on one end at the water level, and a free hanging metal weight on the other end to keep the cable in tension. These systems are most commonly used in surface water applications but are also effective in large diameter shallow wells. They also fall in the middle of this list in terms of relative cost.
Advantages
A float-based water level measuring device is simple, easy to diagnose when things go wrong, and problems that do occur are often easily fixed in the field. Float systems have low power consumption but work best with a small solar panel and battery. Float-based water level measurements systems are relatively inexpensive and simple compared to the other available systems.
Disadvantages
Float-based systems require still, calm water to work reliably. This is often accomplished using “stilling wells,” which are large vertical tubes connected by a horizontal pipe to the body of water you wish to measure the water level of, like a giant site glass tube, so there is a bit of earth moving required. These stilling wells are often seen next to primary measuring structures like Parshall flumes.
That covers the collection of the water level measurements but stay tuned for a future blog about the different systems to transmit the collected data directly to you. If you have a project requiring a water level monitoring system, or any other water resources engineering work, please reach out to me or anyone else on our team at LWS (303-350-4090).
Chris Fehn chris@lytlewater.com
Bruce Lytle bruce@lytlewater.com
